A number of techniques have been disclosed in the prior art for improving the properties, in particular the mechanical properties, of glass fibers. For example, U.S. Pat. No. 2,723,215 describes a process for protecting glass fibers from atmospheric moisture during the fiber-drawing operation which occurs directly after their formation. The technique involves the deposition of a sufficient amount i.e., in the range of 4-30% by weight or more, of a protective coating, to completely coat a yarn formed from these fibers. This protective coating is deposited in the liquid state and is thereafter solidified by cooling the coated material. The composition comprises a base of thermoplastic synthetic or natural waxes which may optionally include other natural or synthetic products of high molecular weight.
U.S. Pat. No. 4,537,610 discloses an alternate embodiment of the technique described above in that a thermoplastic coating is applied to the yarn in a zone located directly below the spinneret which forms the glass fibers. These coated yarns are, however, invariably crushed during their passage through the cutting machine. They must then be reheated up to the softening temperature of the coating composition to enable them to resume their initial shape.
Although the process described in U.S. Pat. No. 2,723,215 is effective in improving the mechanical properties of the coated yarns obtained thereby, it has been found to have a detrimental effect on the thermal and chemical properties of the yarn. In fact, after undergoing the coating process described by the '215 patent, the yarn produced thereby suffers from increased heat sensitivity and has a relatively low thermal stability.